The present invention relates generally to advanced technology thermal battery systems. More particularly, the present invention relates to sodium-sulfur thermal batteries for use in providing a high-power density electrical energy source and most particularly it relates to improved glass sealing systems used in such batteries.
The sodium-sulfur battery was first introduced in the mid 1960's. Since that time, there has been a great deal of interest in developing cell designs which are suitable for a wide variety of applications. Batteries which have been under development include those for use in automobiles and train locomotives. One such battery is described by Frank A. Ludwig in U.S. Pat. No. 4,977,044, the teachings of which are incorporated herein in its entirety. Cell designs have also been investigated for producing batteries for storage of electricity for delayed use in order to level out the production rate of electricity and for space systems requiring high energy density. The sodium-sulfur battery is used as a secondary, that is, rechargeable battery. Its use as a primary (onetime discharge) battery would be unwarranted because of the cost, complexity and fragility involved in edge-sealing and incorporating a ceramic solid electrolyte into a battery design. In addition, there are other relatively inexpensive primary batteries of higher power density available in the marketplace.
Sodium-sulfur thermal batteries typically include a molten sodium electrode, a molten sulfur electrode and a relatively thin sheet of a solid ceramic sodium ion conducting electrolyte which serves as a separator between the sodium and sulfur electrodes. The electrolytic reaction occurs when sodium ions diffuse through the separator to react with the molten sulfur. Consequently, an important requirement for the separator is that it has a sufficiently high rate of diffusion for sodium ions to allow, during initial operation of the thermal battery, the formation of a sodium polysulfide electrolyte within the separator. To provide satisfactory mechanical strength for the thin ceramic sheet separators used in such a structure, it is typically bonded to an underlying support plate comprised of a porous material such as graphite or fused titania.
The sodium-sulfur cell usually operates at a relatively high temperature (300.degree.-400.degree. C.) in order to maintain not only the sulfur and sodium, but also their reaction products, in a molten state. It is believed that the migration of sulfur and sodium into the porous separator occurs when the cell is heated to operating temperatures for the first time to produce a polysulfide gradient. This polysulfide gradient is composed of sodium sulfides having the formula Na.sub.2 S.sub.x wherein x is approximately five or less but greater than or equal to one. The composition of the gradient is believed to be: EQU Na.sub.2 S/Na.sub.2 S.sub.2 /Na.sub.2 S.sub.3 /Na.sub.2 S.sub.4 /Na.sub.2 S.sub.5
Na.sub.2 S is a solid at temperatures below 1000.degree. C. As a result, the solid Na.sub.2 S acts as a barrier which prevents migration of liquid sulfur or sodium through the entire porous separator. At the same time, the remainder of the polysulfide gradient provides levels of ionic conductivity which are not possible with the previous solid ceramic materials. The use of a porous separator in combination with a polysulfide gradient provides suitable liquid electrode separation while also providing high rates of ionic conduction and resulting high electrical power output.
In use, it is found that a structure as described herein above reduces the electrical resistance of the sodium-sulfur cell, leading to low I.sup.2 R power loses and, therefore, high delivered power densities from the battery's cells. In this structure, the porous support structure, by supporting the ceramic separator, allows the use of a separator sheet that would otherwise be too thin to survive the thermal and mechanical stress generated during operation of the cell. At the same time, the porous separator permits sodium to diffuse therethrough and contact the inner side surface of the alumina, so that the cell reaction can occur.
The solid electrolyte is a critical part of the cell configuration because it must provide separation of the liquid sodium from the liquid sulfur in order to prevent catastrophic cell failure. One widely used solid electrolyte in sodium-sulfur batteries is beta"-(double prime) alumina. To improve the bonding between the solid alumina electrolyte and the underlying porous support, one or more glass compositions may be used to join the two materials and seal the edges of the bond line. To be effective, the glass must be able to wet both of the materials and be chemically compatible with the corrosive and high temperature environment of the cell. It must also have a coefficient of thermal expansion which is close (if not identical) to that of the thin and fragile solid alumina electrolyte and also have a viscosity low enough, when molten, to allow fusion to both the porous support and the solid alumina electrolyte at a relatively low processing temperature and yet have a melting point which is high enough to resist deformation over the lifetime of the cell. At the present time, no glass composition available satisfactorily meets all of these requirements.